1. Field of the Invention
The present invention relates to an optical reader (referred to as “scanner unit” hereinafter) adapted for use in a copier, an image forming apparatus, etc., and more specifically to a scanner unit for reading a document by scanning direction which is perpendicular to the read lines and using light emitted by an illumination device to a read line and reflected from the document along an optic axis plane which contains the read line and extends in perpendicular to the document glass table.
2. Description of the Related Art
FIG. 5 is a cross-sectional view of a conventional scanner unit as seen from the front side thereof and FIG. 6 is an enlarged cross-sectional view illustrating the illumination device of the first carriage of the scanner unit in FIG. 5. In the scanner unit 100 shown in FIG. 5, first and second carriages 51, 52 are mounted on a rail (not shown in the drawings) for movement in the scanning direction as indicated by arrow F or R over the range of a predetermined distance in parallel to the document glass table 55 while maintaining a predetermined spaced interval with respect to a document glass table 55. The first and second carriages 51, 52 are driven to move by a wire 53 and a drive pulley 54 which is in turn driven by a belt and a drive motor (not shown). In such a structure of the scanner unit 100, it is so arranged that the first carriage 51 is movable over such a distance that the entire surface of a document 56 placed on the document glass table 55 is scanned from one outer edge SA thereof (or the terminating end of the document as viewed in the arrow direction R) to the other outer edge SZ thereof (or the terminating end of the document as viewed in the arrow direction F).
For the first carriage 51 to read the document 56 placed on the document glass table 55 along read lines LL (imaginary straight lines spaced at an appropriate interval for the reading), an illumination device is provided for illuminating the read lines LL. The illumination device includes an exposure lamp 61 and a reflector 62, and the light reflected from the read lines LL is directed to a carriage mirror 63 along the optic axis plane PP (also called optic axis, referring to a plane that passes through a read lines LL in a perpendicular relation to the document glass table 55). The carriage mirror 63 reflects its received reflected light to the second carriage 52, which reads such reflected light and sends it as data to a lens (not shown) for image formation or other purposes.
In the above-described conventional scanner unit, wherein the document placed on the document glass table is illuminated along the read line LL, the read line LL is illuminated from one side of the optic axis plane PP by direct light from the exposure lamp while it is illumined from the other side by light of the exposure lamp reflected by the reflector. In such a case, the amount of the reflected light (the amount of light passing through a unit area, or brightness) is lessened as compared with that of the direct light because the light directed to the read line LL for illumination thereof by way of the reflector has a longer distance to travel and for other reasons. A difference in the amount of illumination light directed from one side of the optic axis plane PP and the other side thereof causes a problem in the result of image formation. That is, if the document 56 has an attachment portion 56a, as shown in FIG. 7, a shade 56b results from the stepped edge of the attachment portion 56a when the edge of the attachment portion 56a is positioned at a read line LL or coincides with the optic axis plane PP.
Though it may be contemplated to provide exposure lamps on both sides of the optic axis plane for preventing an occurrence of shade, this cannot provide a practical solution in terms of cost because two sets of illumination devices each including an exposure lamp and an inverter must be used and hence the cost of the illumination devices becomes high as compared with the use of a reflector. There has been a proposal to arrange inline type LEDs or the like on both sides of the optic axis plane for illumination instead of exposure lamp and to remove the shade by image processing (Japanese Patent Application KOKAI Publication No. H8-79492), but devices used in such proposed arrangement are complicated and costly and the control program therefor is also complicated.